1. Field of the Invention
The present invention relates to an image reader capable of reading bound documents such as bound files and magazines.
2. Description of the Related Art
Image readers have been commercialized as so-called book scanners. In these devices, an image sensing unit is arranged above a document table, so as to provide an open space several tens of centimeters in height between said document table and said image sensing unit. When reading a bound document (hereinafter referred to as "book document") such as a bound document file or magazine, a user places the open book document on the document table so that the document surface is facing upward. When the start key is pressed, an illumination lamp is turned ON, and prescanning is executed. Operation settings are set based on the information obtained by the prescan, and thereafter a main scan is executed. In the main scan, a read image is sequentially output to an external device. If the external device is a printer, a copy image is formed in real time. The efficiency of multi-page reading operations is increased because the pages of a book document disposed on a document table can be turned when using a book scanner. Furthermore, damage to scanned documents is greatly reduced 47 compared to arrangements requiring face-down orientation of open book documents. In the following discussion, book documents include filed documents.
Unlike a single sheet document, the document surface (i.e., the surface to be read) of a book document is curved. That is, the height of the document surface differs according to the position in a lateral direction (viewing direction). Due to this height differential, distortion occurs in the read image in accordance with the state of curvature and unsharpness of focus. A book scanner measures the state of curvature, and executes focus adjustment and distortion correction in accordance with said measured curvature. Methods of measuring the state of curvature of a document surface include a method of sensing the edge surface at the so-called "head" of a document and analyzing the edge surface image (U.S. Pat. No. 5,585,926), and a slit-projection method using three-dimensional measurement (Japanese Laid-Open Patent Application No. 5-219323).
The quality of the output image is improved by the aforesaid focus adjustment and distortion correction. Conventionally, however, there is the drawback of irregular density in the read image caused by the illumination of the document surface. Irregular illumination of the book document is dependent on the height distribution of the document surface, and cannot be eliminated even by measuring the light distribution of the illumination lamp relative to the flat surface of the document table for correction of reading data (i.e., shading correction) such as is accomplished by sheet-document reading devices such as copiers and the like.